1) Field of the Invention
The present invention relates to an optical device suitable for use with an optical communication system.
2) Description of the Related Art
An optical waveguide device is an optical device which implements various functions by using an optical waveguide for confining light in a region formed in a dielectric medium and having a refraction index to propagate the light therein. For example, an optical waveguide device which configures a Mach-Zehnder interferometer using a dielectric substrate such as lithium niobate (LiNbO3: hereinafter referred to as LN) has a very high electro-optic constant and has a high response speed in comparison with a device which has a thermal-optic (TO) effect. Therefore, the optical waveguide device of the type described is widely used as an optical modulator, an optical switch, a variable optical attenuator and so forth.
However, it is known that, with such an optical waveguide device for which a dielectric substrate of LN or the like is used as described above, a phenomenon called temperature drift that the operating point shifts in response to a temperature variation and another phenomenon called DC drift that the operating point shifts in response to application of a DC signal latently occur. If the operating point shifts in response to occurrence of a temperature drift or a DC drift, then the optical output characteristic of the optical waveguide device varies. Therefore, for example, in the case of an optical modulator, modulation in a normally fixed state cannot be achieved.
In particular, an optical output of a Mach-Zehnder type optical modulator varies in accordance with cos2(Δφ/2). The parameter Δφ in the expression represents a phase variation amount applied by an interacting portion of the Mach-Zehnder interferometer and is represented, in the case of a Z-cut LN substrate, by a relationship of Δφ={π·ne3·γ33·1/(λ·d)}·V, where ne is the refraction index of the optical waveguide, γ33 the electro-optic constant, l the length of electrodes provided on two parallel optical waveguides, λ the optical wavelength, d the distance between electrodes, and V the applied voltage. The optical output characteristic of the optical modulator is represented by such a curve as shown in FIG. 11 wherein the axis of abscissa represents the applied voltage V.
In such an optical modulator as described above, it is desired usually to set the operating point so as to be placed in a middle state between an on state and an off state when the applied voltage between the electrodes is 0 V. However, as seen in FIG. 11, an actual operating point is frequently displaced (shifts) from the desired operating point because of a fabrication error, various stresses and so forth. Against such displacement of the operating point, generally a DC bias is applied to carry out adjustment of the actual operating point to the desired operating point.
However, since the operating point adjusted by a DC bias shifts in response to such a DC drift as described above, in order to achieve stabilization of the operating point, it is necessary to normally monitor an optical output and control the DC bias based on a result of the monitoring. Such monitoring of an optical output as just described is used restrictively only for an optical modulator, but is required, for example, also in a Mach-Zehnder type variable optical attenuator, to adjust the optical attenuation amount in response to a temperature variation or the like.
Japanese Patent Laid-Open No. 2003-233047 (hereinafter referred to as Patent Document 1) discloses a configuration which includes, in order to obtain monitor light of an intensity suitable for use in the bias control described above, a monitoring optical waveguide for guiding part of output light as monitor light and an attenuation section for attenuating the monitor light.
Incidentally, as one of such optical waveguide devices as just described, an optical waveguide device of a butt joint type configuration is known wherein, in order to guide an emitted optical signal to an output optical fiber, an end face of an optical waveguide and the output optical fiber are directly connected to each other. In an optical waveguide device having the butt joint type configuration, for example, as shown in FIG. 12, a fiber fixing member 120 such as a V-groove fiber block or a glass ferrule is used to fix an output optical fiber 110 to an emission end face of an optical waveguide 101A which is formed on a substrate 100 together with an optical waveguide 101B and can output main signal light so that required connection strength of the output optical fiber to the end face of the optical waveguide is secured.
It may seem a possible idea to form such an optical waveguide device having a butt joint type configuration as described above with reference to FIG. 12 such that, in order to monitor the optical output from the optical waveguide 101B, for example, a light reception device 130 for optical output monitoring is disposed on the reverse side of the fiber fixing member 120 (on the opposite side to the optical waveguide device). However, interference disposition between the output optical fiber 110 and the light reception device 130 not only makes it difficult to dispose the light reception device 130 at a position at which monitor light can be sufficiently received but also makes it difficult for the light reception device 130 to sufficiently receive monitor light emitted from the optical waveguide 101B on the monitor side because the fiber fixing member 120 makes as an obstacle.
International Publication No. 2004/092792 (hereinafter referred to as Patent Document 2) disclosed a technique wherein a groove is formed in the proximity of an end portion of a monitoring optical waveguide on the optical output side on a substrate on which an optical waveguide is formed such that light outputted from the monitoring optical waveguide is reflected by a reflecting face provided by a side wall of the groove so that the reflection light is emitted from a side face of the substrate. Consequently, an output side end face of an output optical waveguide to which the output light is guided and the side face of the substrate to which monitor light is guided are configured as different faces from each other so that the interference disposition between the output optical fiber 110 and the light reception device 130 in the optical waveguide device described above with reference to FIG. 12 can be prevented. However, in the technique disclosed in Patent Document 2, while a light reception device such as a photodiode is disposed in the proximity of the device in order to receive monitor light, if the spread in mode diameter of monitor light at a light reception position is small, then it is necessary to adjust the incorporation position of the light reception device with high accuracy.
On the other hand, Japanese Patent Laid-Open No. 2005-345554 (hereinafter referred to as Patent Document 3) disclosed a different technique wherein a side wall of a groove for reflecting light outputted from a monitoring optical waveguide is formed in a convex shape as in the technique of Patent Document 2 described above such that the beam diameter of monitor light reflected from the side wall is spread by diffraction so as to achieve increase of the position adjustment tolerance when a light reception device for receiving monitor light emitted from the substrate side face side is incorporated. For example, in FIG. 4 of Patent Document 3, it is illustrated to increase the spread of monitor light after reflection in a horizontal direction of a substrate face, and, in FIG. 13 of Patent Document 3, a technique is illustrated wherein the shape of a side wall face of a reflection groove in a depthwise direction is formed as a convex shape toward the outside of the groove so that the spread in beam diameter in a depthwise direction by diffraction is provided more effectively than in an alternative case wherein the shape of the side wall face of the reflection groove in the depthwise direction is formed as a straight shape.
However, while, in the above-described technique disclosed in Patent Document 3, the shape of the side wall face of the reflection groove in the depthwise direction is formed as a convex shape toward the outside of the groove so that the spread of the beam diameter in the depthwise direction by the diffraction increases, in such a situation that the depth is limited from a requirement for security of required strength of a substrate, a high technique is required for such a method wherein the shape of a side wall face of a reflection groove in the depthwise direction is formed as a convex shape as described above, and it is not easy to obtain a configuration wherein an expected spread by diffraction can always be obtained.